String Up a Simple, Chaotic Double Pendulum Cat Toy

Dragging a toy on a piece of string is a time-honored way to entertain a cat.The problem is, it may be less fun for you than it is for the cat.

Can the process be automated? After rigorous, exhaustive testing, I have determined that a compound pendulum is the simplest way to do it. Compound-pendulum projects have been published previously in both Make:Volume 22 and Volume 50,but they used motor-driven levers and created their output with LEDs. My version just needs two weights, a piece of string, and an output device commonly known as a “mousie” (Figure A).

Figure A. Illustration is courtesy of Charles Platt

The length of a pendulum determines how quickly it oscillates. Consequently, when you have two weights that are linked together but are hanging at different heights, the movement of the higher weight interferes with the movement of the lower weight, and the result is a series of chaotic movements that cats find interesting.

To keep the pendulum swinging for as long a time as possible, the weights should be as heavy as possible. However, this is a potential hazard: If you build this project, be sure to suspend the weights securely, so that they don’t fall on top of your cat. I cannot take responsibility for feline injuries caused by substandard pendulum construction.

CAUTION: Dangling cords can also pose a strangulation threat to toddlers, so please choose a safe location to mount this project that will not be accessible to small children.

Build the Pendulum

Each weight can consist of a plastic container such as a medium-large vitamin bottle. Drill a hole through the base of the bottle and its cap, and thread some string through the holes. Fill the bottle with small stones, sand, or scrap hardware, then screw the cap on, and tie a large knot under the bottle to hold it securely, as shown in Figure B. Nylon string is stronger, more flexible, and more durable than polyester or natural twine, although knots in nylon are more likely to loosen with time.

Figure B-1. Photo by Tyler Winegarner

Figure B-2. Photo by Tyler Winegarner

I used a piece of fishing line for the bottom section of the pendulum, because this only carries a small load, and fishing line is difficult to see. I figured it would create the illusion of the mousie moving around under its own power, although I’m not really sure that my cat finds this important.

Hang It Up

An easy place to hang the toy is in the opening of a door. Most interior door frames are made of wood, providing a secure location for a #10 pan-head screw at least 1″ long. Use a fender washer under the screw head, to make sure the string doesn’t slip off. And if you don’t want to violate your door frame, you can make a clamp using pieces of hardwood as shown in Figure C, although this will prevent the door from closing.

Figure C. Illustration is courtesy of Charles Platt

If you’re more ambitious, you can suspend the toy from the ceiling. A toggle bolt is a reliable way to do this. You can learn more about toggle bolts, anchors, and other devices for insertion in drywall if you take a look in my book Make: Tools, which has a whole chapter on the subject.

Manage the Motion

Once you’ve suspended your pendulum, you can fine-tune it to make the motion unpredictable. The upper weight should have more momentum than the lower weight, to swing it around. This means the upper weight should be twice as heavy as the lower weight. Experiment by removing some of the mass from inside the lower container till they oscillate in an interesting way.

Your cat may respond most actively when the mousie is close to the floor, like a real mouse. When the cat pounces on the toy and drags it, this renews the motion of the pendulum, so that when the mousie is released, it starts moving again. The interaction can sustain feline interest for 15 minutes or more, and is entertaining to watch. Indeed, you may finally have a cat toy that is almost as much fun for you as it is for your cat.